As shown in FIG. 1, a molten-metal-plating apparatus 20 comprises a container 22 containing a molten-metal-plating bath 21, which may be called plating bath; a snout 23 immersed in a surface portion of the plating bath 21 for preventing the oxidation of a steel sheet W introduced into the plating bath 21; a sink roll 28 arranged in the plating bath 21; a pair of support rolls 27 located above the sink roll 28 in the plating bath 21; and gas-wiping nozzles 26 located slightly above a surface of the plating bath 21. The sink roll 28 does not get an external driving force, but is driven counterclockwise by a friction force by contact with the running steel sheet W. The support rolls 27 are usually driving rolls connected to an external motor (not shown). Incidentally, the support rolls 27 may be non-driving rolls not getting an external driving force. The sink roll 28 is rotatably supported by bearings 90 mounted to the support frames 25. The support rolls 27 are also rotatably supported by bearings mounted to support frames 24. The sink roll 28 and the support rolls 27 are always immersed together in the plating bath 21.
The steel sheet W slantingly enters the plating bath 21 through the snout 23, passes through the sink roll 28, and then changes its course upward. The steel sheet W moving upward in the plating bath 21 is sandwiched by a pair of support rolls 27 pushing the steel sheet W with a constant force, to keep the pass line of the steel sheet W while preventing its curving and vibration. Gas-wiping nozzles 26 blow a high-speed gas to the steel sheet W exiting from the plating bath 21, to make the thickness of a molten metal attached to the steel sheet W uniform by the pressure of a high-speed gas. A plated steel sheet W is thus obtained.
As prior art concerning a bearing assembled in a molten-metal-plating apparatus, JP 2001-207247 A discloses an immersing member coated at least partially with silicon nitride ceramics containing 2-10% by mass of chromium nitride and having a relative density of 95% or more. As shown in FIG. 11, it describes a bearing 90 comprising two semi-cylindrical ceramic bearing members 90a, 90b disposed in a cylindrical metal-made holding member 90c. Because of thermal expansion difference between the ceramic bearing members 90a, 90b and a plating bath (molten metal), the bearing 90 lifted from the plating bath is subject to compression or tensile stress due to the cooling of a plating bath between an outer peripheral surface of the bearing 90 and an inner peripheral surface of the holding member 90c. JP 2001-207247 A describes that stress can be relieved by gaps 90f of 1 mm or more between circumferentially opposing end surfaces 90d, 90e of the bearing members 90a and 90b in the bearing 90.
However, the bearing of JP 2001-207247 A suffers wear and breakage by direct contact between the shaft 90g and the bearing members 90a, 90b, though the breakage of the bearing members 90a, 90b is prevented by the gaps 90f between two bearing members 90a, 90b as described above. Namely, when the gaps 90f between two bearing members 90a, 90b are located such that they are brought into contact with an outer peripheral surface of the rotating shaft 90g, kinetic pressure for supporting the shaft 90g is low at the gaps 90f, resulting in direct contact between the shaft 90g and the sliding surfaces and/or edges of end surfaces 90d, 90e of the bearing members 90a, 90b. As a result, the shaft 90g and the bearing members 90a, 90b are prematurely worn, resulting in shorter lives. Further, in the case of a sink roll driven by a friction force by contact with the running steel sheet without being rotated by a motor, etc., the direct contact of the shaft 90g with the sliding surfaces leads to an increased friction coefficient, so that the rotation of the sink roll cannot follow the running speed of a steel sheet. As a result, slipping occurs between the steel sheet and the outer peripheral surface of the sink roll, likely generating defects such as scratches on the steel sheet.
JP 2001-262299 A discloses a sliding bearing apparatus for a roll used in a continuous molten-metal-plating bath, which comprises a bearing of hard sintered ceramics coming into sliding contact with a shaft of the roll, and a steel-made holder movably holding the bearing. It is described that the bearing is partially cylindrical, with an effective angle of more than 90° and 160° or less. JP 2001-262299 A describes that using a bearing apparatus comprising a holder movably holding a bearing, it is possible to prevent the breakage of the bearing due to the solidification shrinkage of a residual plating metal while the bearing apparatus lifted from the plating bath is cooled.
However, even if the bearing apparatus of JP 2001-262299 A is used, the ceramic bearing may be broken by repeated use. Particularly when the center angle (θ in FIG. 1 in JP 2001-262299 A) of the bearing is relatively large (about 100° or more), the ceramic bearing is highly likely broken.
JP 2002-294419 A discloses a bearing for a roll used in a continuous molten-metal-plating bath, which comprises a metal-made bearing holder, a partially cylindrical ceramic member received in a groove extending in the entire shaft length in an inner peripheral surface of the bearing holder, and a gap provided between an inner wall of the groove of the bearing holder and an outer surface of the ceramic member for discharging a molten metal. It is described that when the bearing is lifted from the bath, a molten metal entering a gap between the bearing holder and the ceramic member can be easily discharged, thereby preventing the compression breakage of the bearing.
However, even if the bearing of JP 2002-294419 A is used, the ceramic member may be broken by repeated use. Particularly when the center angle (θ1 in FIG. 1 of JP 2002-294419 A) of the ceramic bearing is relatively large (about 100° or more), the ceramic bearing is highly likely broken.
JP 2004-530797 A discloses an apparatus for coating a metal ribbon with a molten metal, which comprises a shaft rotatably supported in a molten metal for guiding the metal ribbon, two sliding ceramic bearing shells each having a bearing surface for supporting the shaft, and a deformable element formed by a graphite sheet, etc. between two bearing shells. It is described that because two bearing shells can be deformed while keeping connection, by the deformable element having flexibility, the bearing shells are not broken even if a large radial force is applied between two bearing shells.
However, because two bearing shells are connected via the deformable element in JP 2004-530797 A, a molten metal entering a gap between the bearing shells and a bearing housing containing the bearing shells is not quickly discharged, so that the bearing shells are likely broken due to thermal expansion difference between the bearing shells and the bearing housing and/or the solidification of the molten metal, when the coating apparatus is lifted. Also, because the bearing shells described in JP 2004-530797 A have two flat bearing surfaces (sliding surfaces), stress is concentrated, likely causing breakage.
JP 2006-250274 A discloses a roll bearing used in a continuous molten-metal-plating bath, which comprises a ceramic bearing having an inner peripheral surface for supporting the roll shaft, a metal-made bearing holder for holding an outer peripheral surface of the ceramic bearing, and a buffer disposed between the outer peripheral surface of the ceramic bearing and the inner peripheral surface of the metal-made bearing holder, the buffer being formed by weaving at least yarns with long alumina fibers like a carpet. With the ceramic bearing divided to an upper ceramic bearing part and a lower ceramic bearing part, buffers are disposed between an outer peripheral surface of the upper ceramic bearing part and an inner peripheral surface of the metal-made bearing holder, and between the upper ceramic bearing part and the lower ceramic bearing part. JP 2006-250274 A describes that because the buffers have improved non-wettability to the molten metal, the solidified molten metal can be easily removed, and the molten metal does not enter the buffers, resulting in high heat insulation by air in the buffers.
However, because a gap between the divided ceramic bearing is filled with the buffer in the roll bearing described in JP 2006-250274 A, a molten metal entering a gap between the ceramic bearing and the metal-made bearing holder is not quickly discharged, so that the ceramic bearing is likely broken due to thermal expansion difference between the ceramic bearing and the metal-made bearing holder and/or the solidification of the molten metal, when the coating apparatus is lifted.